To set the issue's assignee from e-mail, set the ccassignee name parameter in the comment handler for the POP/IMAP service used to create issues. Refer to Creating Issues and Comments from Email for more information.
Several 3rd party mail handlers also exist for JIRA that provide features for setting additional fields:
|Name||Compatibility||Fields Supported||License||JRA's||Support Link||Documentation|
|JIRA Enterprise Mail Handler||5.0 - current||ALL, see supported fields||Commercial||see list||link||see website|
|JIRA Advanced Mail Handler||3.7 - 3.12.3||see below||Free||JRA-7316|
Enterprise Mail Handler
JEMH is a fully featured mail handler, that supports all common JIRA fields, as well as complex types like cascade selects. Directives to set fields can be supplied in many ways through Field Processors, e.g. subject, body, XML, CSV, regular expression matching, SMTP mail X-header values. JEMH configuration is now fully UI based, and includes TestCase features to validate configuration.
Examples of common formats come bundled in the plugin (generate examples option in TestCases section). See also the wiki.
Advanced Mail Handler
The allows users to define fields for the issues created email such as reporter, issue type, priority, summary, description and more.
The priority can be set on a scale 1-5 by setting the X-Priority on the email itself. See your mail server's documentation for tips on how to set the X-Priority. A related discussion appears in JRA-7316.